In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. In many fuel systems, when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of injection. However, these conventional injectors rely on injector or system components upstream of the nozzle assembly to determine the injection timing, metering and rate shape, and, therefore, may not provide the optimum control over the fuel injection event necessary for certain applications and to achieve certain objectives.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by optimizing the fuel injection timing, metering and injection flow rate for a particular application or set of operating conditions. For example, emissions may be minimized by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. In other applications, pilot and multiple injections produce the optimal combustion event. As a result, many closed nozzle assemblies have been proposed for enabling more precise control of injection timing, quantity and flow rate throughout engine operation.
One way of more precisely controlling the movement of the needle valve element of a closed nozzle element and, therefore, more precisely controlling the fuel injection event, is to utilize a piezoelectric actuator. U.S. Pat. No. 4,649,886 to Igashira et al. discloses a piezoelectric actuator controlled fuel injector where the amount of fuel delivered by the operation of the injector is determined by the driving voltage applied to the piezoelectric actuator. The actuated piezoelectric actuator acts upon a piston which compresses the fuel inside a pump chamber, wherein the compressed fuel is supplied to an injection valve. The reference further discloses that the injection valve includes a needle valve having a step-shaped portion that includes a small diameter portion under a larger diameter portion. The pressure of the compressed fuel acts upon the stepped portion of the injection valve to overcome forces biasing the valve shut thereby raising the needle valve to open the nozzle of the injector. However, the injection fuel, metered by a check valve, is used lift the needle valve to the open position and this metered fuel is then injected from the nozzle. Therefore, the opening of the needle valve element and, therefore, the timing of the injection event is undesirably dependent on the pressure of the fuel to be injected. Moreover, it has been found that the piezoelectric actuators are incapable of effectively and efficiently generating the high fuel pressures desired in many fuel system applications.
U.S. Pat. No. 4,728,074, No. 4,784,102, No. 4,909,440 and No. 5,452,858 and PCT Publication No. WO 96/37698 all disclose fuel injectors which utilize a piezoelectric actuator to relieve pressure in a chamber so as to cause a needle valve element to open. For example, U.S. Pat. No. 5,452,858 discloses the use of a piezoelectric actuator to drive a piston which changes the pressure of a working fluid, separate from the injected fuel, in a pressure chamber to control the opening and closing of a needle valve. However, the injectors disclosed in each of these references disclose that the piezoelectric actuator is energized to expand a pressure chamber located adjacent to the injector needle, thus decreasing the pressure within the pressure chamber, in order to relieve the forces biasing the injector needle closed. Also, this injector is not fuel pressure balanced in the closed position and thus the piezoelectric stack be maintained in the expanded state to maintain the hydraulic pressure in the pressure chamber at a high level to hold the element in the closed position.
PCT Patent Publications WO 93/06625 and WO 94/19598 each disclose fuel injection valves using a piezoelectric actuator for moving a piston to controllably vary the pressure of fluid in a hydraulic chamber which is fluidically separate from a fuel supply. The hydraulic chamber is positioned at one end of a needle valve element biased by a spring toward the piezoelectric actuator into a closed position. Pressurization of the fluid in the hydraulic chamber forces the needle valve element into an open position to begin injection of fuel supplied to a nozzle cavity. However, each of these injection systems requires the needle valve to be an outwardly opening valve. Also, the needle valve elements do not appear to be fuel pressure balanced in both positions.
Consequently, there is a need for an improved closed nozzle fuel injector assembly operated by a piezoelectric actuator which is capable of effectively, precisely and selectively controlling the rate and degree of opening of a needle valve element.